Quantum effects in the diffusion of hydrogen on Ru(0001)

TL;DR

This study uses helium spin-echo measurements and quantum transition state theory to investigate hydrogen diffusion on Ru(0001), revealing quantum tunneling effects up to 200 K and a tunneling-dominated rate below 120 K, with implications for catalysis and quantum modeling.

Contribution

First combined experimental and theoretical investigation of hydrogen tunneling on Ru(0001), demonstrating quantum effects at relatively high temperatures.

Findings

Quantum tunneling dominates hydrogen diffusion below 120 K.

Measured jump rate of 1.9×10^9 s^-1 at low temperatures.

Theoretical models reproduce high-temperature behavior but underestimate tunneling rates.

Abstract

An understanding of hydrogen diffusion on metal surfaces is important, not just for its role in heterogeneous catalysis and hydrogen fuel cell technology, but also because it provides model systems where tunneling can be studied under well-defined conditions. Here we report helium spin-echo measurements of the atomic-scale motion of hydrogen on the Ru(0001) surface between 75 and 250 K. Quantum effects are evident at temperatures as high as 200 K, while below 120 K we observe a tunneling-dominated temperature independent jump rate of 1.9$\times$10$^9$ s$^{-1}$, many orders of magnitude faster than previously seen. Quantum transition state theory calculations based on ab initio path-integral simulations reproduce the temperature dependence of the rate at higher temperatures and predict a crossover to tunneling-dominated diffusion at low temperatures, although the tunneling rate is under-estimated, highlighting the need for future experimental and theoretical studies of hydrogen diffusion on well-defined surfaces.